Tip #15: Check engine light with oxygen sensor code for 94 BMW

If you have a check engine light in your 1994 BMW with codes 12 or 13, it means the oxygen sensor circuit is faulty. Don’t rush and replace the sensor right away but test the oxygen sensor voltage reading to confirm if it is really defective.

Symptoms:

Check engine light in the dash is always on
Engine runs rough despite changing the spark plugs and wires.

Oxygen sensor monitors the fuel mixture condition of your engine. The info gathered is fed to the engine computer which then decides the amount of fuel to be injected to your injectors. If the info says lean, the computer will command the injectors to spray more fuel and vice-versa.

Testing:

You can measure the voltage of the oxygen sensor while the engine is running by hooking up a voltmeter on the sensor’s signal wire. The reading should be oscillating from 0.1 volt to 0.9 volt. To test, spray some carb cleaner to your throttle plates and the reading should stay in the upper range (say around 0.7 to 0.9 volt). You can also unplug any vacuum line to allow more air causing lean mixture and this will cause the reading to hover in the lower range like around 0.1 to 0.3 volt. If the sensor is not reacting when you do these, the sensor is bad. Otherwise, look for the causes of the oxygen sensor code like vacuum leak, gasket leak, sensor wiring problem and etc. More detailed test is available if you are a member of ATS including later models of BMW.